Telephone line finder circuit



July 27, 1954 THOMPSON 2,684,997

TELEPHONE LINE FINDER CIRCUIT Filed Nov. 15, 1951 2 Sheets-Sheet l BERTH. THOMPSON BY ya 6 c ATTORNEY B. H. THOMPSON TELEPHONE LINE FINDERCIRCUIT July 27, 1954 Filed Nov. 15 1951 2 Sheets-Sheet 2 DIFFERENT/ALMARG/NAL INVENTOR BERT H. THOMPSON Patented July 27, 1954 UNITED STATESTELEPHONE LINE FINDER CIRCUIT Application November 15, 1951, Serial N 0.256,547

Claims.

This invention relates to telephone systems and particularly toautomatic telephone systems havtems having coin-controlled lines.

The object of the invention is to minimize double connections and lossof revenue on coincontrolled lines in an automatic telephone system.

Coin-controlled lines in automatic telephone systems are well known. Anexample of such a line widely employed in a large telephone system isthat described in Patent 1,569,623 granted to B. G. Dunham, January 12,1926 and another is that described in Patent 2,427,000 granted to H.

Hovland, September 9, 1947. Such lines and others are subject to doubleconnections and loss of revenue which the present invention is designedto prevent.

The invention and its features may be understood by considering a systemin which the invention is presently embodied, one such system beingshown in the accompanying drawing and described in detail hereinafter.It is to be understood, however, that the invention is not limited inits application to the specific system disclosed herein but is generallyapplicable to automatic systems comprising coin stations.

In the drawings:

Fig. 1 shows a subscriber telephone station, equipped with a coincontrol, connected through a conventional line finder indicated by theline finder convention to the left-hand portion of a coin trunk; and

Fig. 2 shows the right-hand portion of the coin trunk which is connectedthrough a selector, indicated by the selector convention, and aconnector to a subscriber telephone station.

The invention is illustrated in an automatic telephone system in whichswitches of the stepby-step type are used for extending connectionsbetween calling and called subscribers lines. Reference may be had topages 53 to 65 of the second edition of Automatic Telephony by Smith andCampbell for a description of the structure and operation ofstep-by-step selector and connector switches; and reference may be hadto the patent to R. L. Stokely, No. 1,799,654, granted April '7, 1931for a description of a line finder switch of the step-by-step type.

The subscribers stations A and B are of the type usually provided with acoin-control mechanism comprising the coin actuated contact and the coincollect and refund magnet. Reference may be had to the patent to O. F.Forsberg, No. 1,043,219 of November 5, 1912 for a description of apre-pay coin box,

Originating a call The manner in which the system disclosed in thedrawings functions when a call is originated at station A will now bedescribed. In the following description of the functioning of thesystem, it is assumed that the positive side of the central oiiicebattery is grounded. Th refore, wherever the word battery is used, it infact denotes the negative side of the battery; and where the word groundis used, the positive side of the battery is meant.

When the receiver at station A is removed from normal position, a linefinder LFI is operated in a manner described in the aforementionedpatent to R. L. Stokely to connect the line Ll to the coin trunk. Uponconnection of line LI through the brushes of line finder switch LP! tothe coin trunk, a circuit is established to operate relay 53 in Fig. 2.The circuit may be traced from negative battery through contact Ifib,conductor 16, contact 20a, resistance 42, conductor H8, contact 50b,bottom winding of relay 53, conductor H3, contact 2| (1, conductor 12,contact I l contact Me, conductor I2, contact LFIb, lower loop conductorof loop LI, left-hand coil of inductance I0, left-hand receiver hookcontact, transmitter 9, dial contact 80, upper loop conductor of loopLi, contact LFIa, conductor I I, contact Mb, contact I do, conductor 13,contact Zia, inductance coil 22, conductor H2, top winding of relay 53and contact 50c to ground operating relay 53. The operation of relay 53closes a circuit from ground through contact 53a, winding of relay 52,conductor H0 and contact 206* to battery operating relay 52 andestablishes also a parallel circuit through contact 500, conductor i II, contact 55a, winding of relay i5, conductor H and contact 25c tobattery operating relay I5. The operation of relay I5 by opening contactI51 opens a short circuit around inductance coil M. The operation ofrelay I5 connects ground through contact l5e and conductor Is to theline finder sleeve circuit to hold the line finder in a well-knownmanner. The operation of relay 52 establishes a circuit extending fromground through contact 52b, winding of relay 50, conductor I10 andcontact 296 to battery operating relay 50. The operation of relay 5!]closes a circuit to selector SI. The circuit may be traced from the tipconductor of selector SI through contact 5m, conductor IilI, resistance35 and the winding of polar relay I I in parallel, junction ll,inductance coil 4 I, conductor 14, contact '21 conductor 82, resistance40, conductor 08, contact 509, conductor I03, resistance 38 in parallelwith the winding of polar relay i8, conductor Hi6, and contact die tothe ring conductor of selector Si. This establishes a bridge across thetip and ring conductors of selector Si and actuates the line relay, notshown, in the selector in a well-known manner. The circuit was shown toextend through the windings of polar relays ii it. When the circuit isidle the armatures of polar relays i? and it are spring biased to thepositions shown in Fig. 1. They are arranged so that the armature of oneor the other of them is actuated to its opposite contact in response tocurrent of a particular polarity. In response to the polarity of currentimpressed on them for the present condition the armature of relay i8 isactuated. to engage contact 58a and the armature of relay ll remains inengagement with contact {lb due to its spring tension. The operation ofrelay 55 provides a holding ground circuit for relay at contact 56dwhich replaces ground at contact 530; when contact Sec opens. Theoperation of relay 5% by opening contact 5% and closing contact 5M alsochanges the circuit, heretofore traced, which supplied battery to thering conductor I? through resistance Hi2, so that the battery is nowsupplied to the conductor H! from the same source at contact 55b throughcontact 23a, winding of relay is, conductor HS, top winding ofdifferential marginal relay 55, contact Bea and the bottom winding ofrelay 53 to conductor H3 which has been shown to connect to ringconductor l2. The operation of relay 5G, by opening contact 556disconnects ground from the right-hand terminal of the upper winding ofrelay 53 and ground through the middle winding of relay 55 issubstituted therefor. Relay 53 is maintained operated but relay 5'5which responds only to a heavier than normal ground does not operate atthis time. The operation of the selector relay as a result of theconnection of the bridge across the tip and ring conductors of theselector also connects ground, not shown, to the selector Si sleeveconnection and through conductor HT, contact lied and conductor E3 tosupplement the line finder sleeve holding circuit. In response to theconnection of the bridge across the tip and ring conductors of theselector the selector impresses dial tone on a circuit which extendsfrom the tip of the selector Si through contact Sic, conductor 59!,resistance 35 and the winding of relay ii in parallel, resistance 3contact 55b, condenser 35, contact Mb, conductor 5 i, LF: contact a,upper loop conductor, condenser 5. right-hand coil of inductance Iii,winding of the receiver, dial contact 8a, right-hand and lefthandcontacts of the receiver hook, left-hand inductance coil, lower loopconductor, LFi contact I), conductor 22, contact l le, condenser 31,contact i5c, resistance flit, resistance 38 in parallel with winding ofrelay it, conductor H28 and con tact 510 to the ring of selector Si. Thetone thereby produced in the receiver is a signal to the subscriber todeposit a coin.

Coin deposited The ring of the loop, as has been shown, connects tobattery and the tip connects to ground. When a coin is deposited in thecoin control device, contact 6 is closed which establishes a path Pwhich is now operated since the resistance of the loop is now reduced bythe parallel path to ground through contact 5. The operation oi relay isestablishes a circuit from battery through contact isa, winding of relayiii, conductor i is and contact 56b to ground, not shown, through thesleeve of the selector circuit and in parallel through conductor ii'i,contacts 223d and [5e to ground, operating relay it which looks tobattery through contact 560. Relay is operated establishes a circuitfrom battery through contact it to parallel paths, through the windingof relay iii to ground and through contact 21c and the winding of relay23 to ground operat-' ing relays 24 and 2G. The operation of relay italso establishes a circuit from battery through the winding of relay 23,conductor 8|, contact led, conductor Ti, and contact 21d which extendsto the ring of the loop, so that now the winding of relay 23 is inseries with the ring of the loop, replacing the path from batterythrough contact ifib, the winding of relay is and the lower winding ofrelay 53 which path was opened at contact it!) when relay is operated.Relay i9 releases but relay 53 is held operated for a short intervalnotwithstanding the opening of the path through its lower winding, asits upper winding is still energized from the new battery sourcesupplied around the loop through the winding of relay 23. The operationof relay 23 establishes a circuit from ground through contact 2312 andthe winding of relay &5 to battery operating relay 46. The operation ofrelay 23, at contact 23a, closes a shunt around a portion of theselector bridge circuit, which shunted portion includes conductor E88,contact 50g and conductor His, so that the bridge will be maintainedafter relay 5i) releases. Relay operated, by opening contact 45a andclosing contact 1161) supplies ground from contact 231) to the windingof relay [5, to replace the ground supplied through contact 59d andconductor ill after relay releases, so as to maintain relay it operatedafter the release of relay 50. Relay 2i when operated supplies batterythrough contact 240. to take over control of relay 26 from relay [6through contact IEf. When relay 20 operates relays 5i! and 52 arereleased by opening contact 296 which supplied battery to relays Eli and52 over conductor H0. The operation of relay 2!] also separates thesleeves of the line finder and selector which were joined throughcontact 2903. By closing contact 230, relay 26 connects ground to thesleeve conductor it of the line finder to supplement the ground fromcontact 56. The release of relay 52 short-circuits the top winding ofrelay 53, releasing relay 53, by connecting ground through contact 52a,conductor Hi9, contact 56c, conductor i8 and conductor M2, to theleft-hand terminal of the top winding of relay 53, the right-handterminal of the top winding being also grounded. Relays 5t and 52 areslow in releasing in order to insure that relay 23 is fully operatedbefore relay so releases and opens the bridge to the selector in caserelay 53 releases when the coin is deposited.

The purpose of differential marginal relay 58 is to block the operationof the coin trunk in case of an irregular condition on the line. In response to such a condition relay 5% operates and prevents furtheroperation of the trunl: in the following manner.

When relay 56 operates, due to variations from the normal currentconditions, which normal current conditions maintain the armature of therelay in engagement with its lower contact 5611, contact 55a is closedand relay 55 is locked in this position over a circuit from batterythrough the bottom winding of relay 56, resistance 65 and contact 56a toground in the selector sleeve circuit. The function of resistance 65 isto prevent overheating of relay 56. In order for dialing to proceed, ina manner to be described, it is necessary for relay I6 to be operated ina manner heretofore explained over a path which extended through contact56b. With contact 56b opened relay [6 cannot operate and, since dialingcannot proceed, a desired party cannot be connected through the cointrunk. The circuit will remain in this condition and it is necessary forthe calling party to hang up his receiver, after the irregular conditionis terminated, before a call can be successfully placed.

Dialing The calling party now actuates calling dial 8 in a well-knownmanner, opening and closing contact 80 a certain number of times foreach digit dialed corresponding to the number of the digit. In responseto this the winding of relay 23, which is connected in series with theloop, as has been shown, is energized and deenergized and selectorbridge contact 23a is opened and closed a corresponding number of timesfor each digit dialed to repeat the dialed pulses to the selector. Relay46, controlled by relay 23, is released in response to the first openingof contact 23b and, since it is slow in operating, relay 45 remainsreleased during the pulsing of each train of pulses after the firstpulse of each train. Relay I5 is in turn controlled by relay 46,receiving its battery through contact 461). Relay I5 is thereforereleased during the dialing of each train of pulses so as tominimize-inter ference with the dialing by short-circuiting theinductance 4! at contact I5).

The selector SI is actuated vertically, in response to the reception ofthe first train of pulses corresponding to the first digit, a number ofsteps corresponding to the number of the digit and then automaticallymoves horizontally at the vertical level corresponding to the number ofthe first digit until it identifies an idle trunk or succeeding switch.It will be assumed that terminals SI are thereby selected, extending thetip, ring and sleeve terminals of the selector through terminals Sic,Sit) and Sic, the tip conductor and the top winding of relay 58 toground. The ring conductor is extended through contact tile, and thebottom winding of relay 58 to battery operating relay 58, whichbyclosing contact 58a in turn operates relay 59. Relay 59 impressesholding ground potential through contact 59a, and selector contact Sicon the sleeve circuit of the coin trunk. The following two digits dialedby the calling party actuate repeater relay 23 of the coin trunk and arein turn impressed on relay 58 of the connector. In response to this theconnector CI is operated to select the called line. Connector CI makes abusy test of the line and if the line is not busy impresses ringingcurrent on the called line to call the called subscriber.

Called party responds It will be assumed that connection has beenestablished to the called line at SUB station B through connectorterminals Cla, CH2 and Clc and that the called party responds to theringing by lifting his receiver which connects the loop tip and ringconductors together. This closes a circuit from battery through thelower winding of relay 5'! around called subscriber Bs loop and backthrough the top winding of re lay 57 to ground operating relay 51. Thisopens contacts 51a and 51c and closes contacts 51b and 5701. Battery andground which before the subscriber answered were connected through themiddle and top conductors of the coin trunk respectively are nowconnected through the top and middle conductors respectively. Thisreverses the polarity of the current through the windings of relays I1and I8. The armature of relay I! is now actuated to engage its contactIla in response to the changed direction of How of current through itswinding and simultaneously the armature of relay I8 is actuated due toits spring tension to engage its contact I8b. This establishes a circuitfrom ground through contact 20 contact I 8b and contact I la to parallelbranches. One branch extends through contact 21a and the winding ofrelay 21 to battery operating relay 2?. The other branch extends throughconductor I I5, contact 55d, contact 54a, winding of relay 5| andresistance 66 to battery operating relay 5|. Relay 21 looks through itscontact 21b to ground through contact 20f. Re lay 5i looks through itscontact 5th and conductor I05 to the same ground on contact 20f. Relay5| operated in turn operates relay 55 over a circuit from batterythrough the winding of relay 55, contact 5 lg, contact 5 In, conductorI65, and contact 20 to ground. Relay 5I operated reverses the tip andring conductors from the selector so that they are now connected to thering and tip conductors respectively of the coin trunk through contacts5i?) and 5Id, instead of through contacts 5m and 5| 0 as heretofore, tothe tip and ring conductors respectively of the coin trunk. Theoperation of relay 5i also short-circuits the Winding of relay I! aswell as resistance 35. The circuit may be traced from the bottomterminal of the winding of relay I1, and the right-hand terminal ofresistance' 35, at junction 89, through conductor I GI, terminal 90,contact 5Ie, contact 54c and conductor I02 to junction 9! of the topterminal of the winding of relay I1, and left-hand ter minal ofresistance 35, shorting out resistance 35 and the winding of relay II.The armature of relay I? therefore disengages from contact I'Ia and isrestored by spring tension to engage contact i'Ib. Due to the operationof relay BI and the reversal of the battery connections on the tip andring conductors, the armature of relay I 8 now is actuated to engage itscontact I 8a. This establishes a circuit from battery through resistance64, winding of relay 54, contact 55a, conductor IM, contact la andcontact 25f to ground operating relay 54. Relay 5d operated locksthrough contact 540, conductor Hi5 and contact 20 to ground. Theoperation of relay 54 removes the short circuit from around the windingof relay IT by opening contact 54c, through which the short-circuitingpath was traced. If due to an irregular operation at the called station,such as the actuation of the receiver switchhook or acctdental actuationof the dial, relay 5'! is released before relay 54 is operated, due tothe reversal of the polarity of the battery caused thereby, asexplained, the armature of relay I 3 will be restored by its springtension to engage contact I 85. Under this condition relay 54 willnevertheless be operated over: the same operating path except that itextends through both contacts Ill) and I81) to the same ground atcontact 20).

Preparing to collect coin Relay 2'! operated establishes a circuit frombattery through contact 256, conductor 7 I, winding of relay I4,conductor Ill, contact 21d, conductor I ll, contacts Sic and contact 58ato ground operating relay I4. The operation of relay 2? also prepares acircuit for collecting the deposited coin by closing positive coincontrol battery through lamp 28, contact 210 and the winding of relay 25to the open contacts 2H) and 2Ic. Relay I4 operated disconnects the tipand ring circuit of the calling subscribers loop from the inductancecoil 22 and pulsing relay 23 which releases. The dial tone circuitthrough condensers 3t and 31 is also disconnected from the callingsubscriber's loop. By closing contacts I lc and Mel relay I4 connectsthe subscriber loop to conductors [Ill and I136 which extend through thewindings of relays I! and I8 to the first selector. When relay 23releases, it in turn releases relay It by opening contact 2312. Relay 45in turn releases relay I5 by opening contact 45b. The release of relayI5 performs no useful function at this time.

The talking battery circuit for the calling subscribers station at thepresent time may be traced from battery through the lower winding ofrelay 58, and since relay 57 is operated, as the called subscribers loopis closed, through contact 5%, contact SIa, contact 5Ib, conductor H35,resistance 38 and the winding of relay iii in parallel, contact Md,conductor I2, contact LFIb, lower loop conductor, left-hand coil ofinductance ID, left-hand contact of the receiver hook, transmitter 9 anddial contact 80 to two parallel branches to ground. One of the twoparallel branches extends through the winding of coin magnet I andcontact 6 to ground. The other extends through the top loop conductor,contact LFIa, conductor II, contact I lc, resistance 35 and the windingof relay I! in parallel, conductor iIiI, contact 5Id, contact SIb,contact 510! and the top winding of relay 58 to ground. It will be notedthat the winding of relay I1 is in one of the parallel branches withground connected to its right-hand terminal. For this condition thearmature of relay I1 is maintained in engagement with contact I'Ib underspring tension. The calling and called subscriber may now converse.

Call not charged If the line current is not reversed relays I'I, 5|, 2?,55 and 54 will not operate and the circuit remains in a condition toreturn the coin when the calling party disconnects. On this type oi callrelay I4 does not operate, condensers 3E and 31 in the tip and ringleads are in the talking circult and relay 23 and inductance 22 areconnected in the battery and ground paths.

Disconnect When the calling subscriber disconnects, the line finder isstill held operated by ground through contact 200. When the callingsubscribers loop is opened by the opening of the receiver switchhookcontacts, relay 58 releases as its top and bottom windings are connectedin the loop circuit as heretofore traced. The release of relay 58 openscontact 580., in turn releasing relay 591. The opening of contact 59aopens the ground connection supplied through contact SIc to parallelbranches. One branch extends through contact 5% and conductor IIB to thewinding of relay I6 which releases. The other branch extends throughconductor II'I, contact 21d and conductor ID to the winding of relay I4which releases. Relay 25 operates over a path from ground throughcontact IZBa in the alarm circuit, contact 269, contact 2522, winding ofrelay 25, conductor 19 and contact I69 to battery. The operation ofrelay 25 establishes a circuit from battery through contact 25 filamentof lamp 29 in parallel with resistance 43 and the Winding of relay I34in the alarm circuit to ground, lighting lamp 29. A circuit may betraced from battery in the alarm circuit through conductor I33, contact20h, conductor I32, winding of relay I25 in the alarm circuit throughinterrupter I21 to ground operating relay I26 intermittently,alternately grounding conductors I24 and I25 through contacts I26a andI261), respectively, for equal intervals which may, for instance, be ofone-half second duration each. This feature is employed to cause thecoin control battery of this circuit, to be described later, to beconnected to the subscribers line for one-half second duration andthereby insure sufi'icient time for proper operation of the coin controlmagnet. Relay 25 operated locks through its contact 250 and contact 28fto ground. Relay 25 operated also establishes a circuit from batterythrough contact I52), conductor I5, contact 2012, contact 250, windingof relay 2| and contact I251) to ground when interrupter I2! is inproper position.

If the called subscriber should hang up before the calling subscriber,relay 51 releases, reversing the polarity of the battery to the tip andring conductors or" the calling loop. In response to this, since batteryis now connected to the right-hand terminal of relay Il, the armature ofrelay I? is actuated to engage contact Ila. The armature of relay I8 isactuated due to its spring tension to engage contact I81). Thisestablishes a short circuit around the winding of relay 5i. It will berecalled that relay 5! was locked over a path from battery throughresistance 66, winding of relay 5 I, contact 5Ih, conductor I05 andcontact 20) to ground. With contacts Ila and I8b closed, a path may betraced from battery through resistance 66, contact 5419, contact 550,conductor H5, contact Ila, contact I81) and contact 23 to ground. Thispath passes through junctions with the upper and lower terminals of thewinding of relay 5I short-circuiting the winding. The resistance. 65 isa current limiting resistance for the shortcircuit condition. Relay 5Ireleases in turn releasing relay 55 by opening contact 5Ig. The windingof relay I? and resistance 35 are now short-circuited over a path fromjunction 9i of the left-hand terminal of the resistance and upperterminal of the winding through conductor I82, contact 54d, contact 5 Ifto junction which connects through conductor IBI to terminal 89 of theright-hand terminal of resistance 35 and the lower terminal of thewinding of relay H. The armature of relay I? is actuated under springtension to engage its contact I'Ib. With the armature of relay I8 inengagement with its contact ISa and relay 55 released, the locking pathfrom battery through resistance 54, winding of relay E l, contact 540,conductor I55 to ground through contact 20 is shorted over a path frombattery through resistance 64, contact 551), conductor I I4, contact la.and contact 20 releasing relay 54. Resistance 64 is a current limitingresistance for this short-circuit condition. With relay released thewinding of relay I1 is again connected on the ground side of the line.If the calling party disconnects first, relays 5i 55 and 54 release whenated condition. When the calling station disconnects relay 23 releases,releasing relay 46 and relay l5, and opens the loop to succeedingswitches.

After an interval, ground is removed from the sleeve circuit Slc of theselector. From this point forward the circuit functions as described inthe preceding paragraph except that relays 5 I, 55 and 54 will not havebeen operated.

Coin control The operation of coin control relay 21 connects positive ornegative coin battery to the line to collect or return the coin. If thecoin is to be collected, positive battery is connected through thefilament of lamp 28, contact 210, winding of relay 26, contacts Zlb andZlc in parallel, conductors l3 and 12 through the tip and ring circuitof the calling subscriber loop in parallel, through the winding of thecoin magnet and contact 6 to ground. This actuates the coin mechanism ina direction to collect the coin. If the coin is to be returned, negativecoin battery is connected through the filament of coin return lamp I28,contact 2U, contact 21c, winding of relay 26 and contacts 21b and 2Ic inparallel over the same path to return the coin. Relay 26 operates inseries with the coin magnet and remains operated as long as the coincollect or coin return battery is connected to the line.

The positive coin collect battery, if the coin is to be collected, ornegative coin return battery, if the coin is to be returned, whenconnected to the line should dispose of the coin, but the magnet willhold ground on the tip of the line as long as it is operated. Theoperation of relay 26 connects battery through contact 26a to thewinding of relay 24 to hold relay 24 operated, since the operatingcircuit of relay 24 is opened when relay 2| operates. When interrupterrelay I26 is released and contact i261) is opened to disconnect ground,relay 2| releases, opening contacts Elb and 210, removing the coincollect or coin return current, releasing relay 26 and holding relay 24operated. At the next closure of ground through contact I261), relay 2!reoperates connecting coin collect or coin return battery again to theline. The coin should have been collected or returned on the firstapplication of potential which upon the removal of the positive ornegative potential restores the coin magnet to normal and no currentshould fiow through the winding of relay 26 on the subsequentapplication of coin potential.

Release of circuit It was explained in the foregoing that before thecoin is disposed of when relay 2| is operated, relay 26 is operated toground through contact 6 in the coin control device. The coin should bedisposed of on the first operation. If the coin is not disposed of relay26 will reoperate each time ground from contact 5262) operates relay 2!.In this event, battery from contact 26a will hold relay 24 which in turnwill hold relay 20. Thereupon battery will be supplied through contact15b, conductor 16, contact 2012, contact 25d, filament of lamp 29 andresistance 43 in parallel through the winding of relay I34 to groundoperating an alarm through contact H3400 after a predetermined intervalin a well-known manner.

After the coin is disposed of the ground through contact 5 is notavailable. Relay 26, therefore, does not reoperate when relay 2!reoperates. Since battery is disconnected from the winding of relay 2dboth at open contact 212, relay 2! being operated and at open contact26a, relay 25 being unoperated, relay 2s releases. This in turn releasesrelay 2%} by disconnecting battery through contact 24a from the windingof relay 2d. The release of relay 2%, by opening contact 28c,disconnects ground from the sleeve of the line finder and releasesrelays 2| and 25 by opening contacts 2th and 26 respectively. Theopening of contact 253] also releases relay 2'! which was looked throughcontacts 271) and 20 and releases relays 55 and 54, and releases alsorelays 2'5, 55 and 54 if operated at the time. The release of relay 2%]also reconnects battery through contact [6b and contact 26a to the paththrough resistance 42 and the bottom winding of relay 53 to restore thecondition prevailin at the outset as relay 21 releases to connectconductors l3 and 12 to the top and bottom windings of relay 53,respectively. The opening of contact 26b removes battery suppliedthrough contact I52), filament of lamp 2%! and resistance 43 in paralleland the winding of relay its in the alarm circuit to ground. The openingof contact 2th opens the path through the winding of interrupter relayI26 in the alarm circuit. With relay 2i released the path through thetop and bottom windings of relay 53 is reestablished. Thus the circuitis restored to the normal idle condition.

Calls to special service operator on calls to a special serviceoperator, the selector level trunk is so arranged that battery andground are reversed immediately when the operators anwers. This causesthe circuit to function as described in the foregoing under the headingCoin Deposited. If coin collect current is supplied to the line from thetrunk, the armature of relay l'i remains in engagement with contact Nband the armature of relay is is actuated under sprin tension to engagecontact lBb. If coin return current is applied to the line from thetrunk, the armature of relay I1 engages contact [la and the armature ofrelay l8 remains operated. Neither relay performs a useful function atthis time.

On a rering by the special service operator, relay Il may operate onringing current. If relay il remains operated due to earth potentialwith relay l8 released it shunts down relay 5!. Relay 5! released,releases relay 55 and short-circuits relay H. The earth potential whichmay have held relay ll will not operate it after the short circuit isremoved. The release of relay 5| also reverses the tip and ringoperating relay I 3 which shunts down relay 5i.

Preventing double connections Double connections on line terminals ofline finders associated with presently known coin box trunks occur durinmomentary opening or'the sleeve holding ground from the coin box trunk.During the instant that the ground is opened, a battery connected to thesleeve from the present coin box trunk is left unguarded. The unguardedbattery has been found to cause double connections under certainconditions. This is prevented 11 in the arrangement of coin box trunkherein by a redesign of the sleeve circuit of the trunk and itsassociated connections which make it impossible to produce an unguardedbattery condition on the sleeve at any instant durin operation.

When double connections occur, frequently the call is completed throughthe first connection while the second connection finds the line busy asa result of the first connection. One coin trunk attempts to collect thecoin while the second attempts to return it. If the second is successfulrevenue is frequently lost. This is prevented in the arrangement herein.

What is claimed is:

1. In a telephone system, in combination, lines and stations, asubscribers prepay coin station circuit connected to one or" said lines,said station circuit comprising a coin control mechanism, a switchhookand a dial, means including a line finder and a coin control trunkcircuit cooperatively responsive to said mechanism, said switchhook andsaid dial for establishing connections between said one line and acalled subscribers line, a sleeve connection in said line finder, asleeve circuit in said coin trunk connectable to said sleeve connection,a first relay control in said trunk for said sleeve circuit, said firstcontrol operable in response to the seizure of said trunk by said linefinder to connect direct ground from said sleeve circuit to said sleeveconnection, and a second relay control in said trunk responsive to theprogress of said call through a succeeding stage, for connecting asecond direct ground in said trunk in parallel with said first groundcontinuously until after said prepay circuit disconnects, to preventdouble connections and loss of revenue.

2. In a telephone system, in combination, lines and stations, asubscribers prepay coin station circuit connected to one of said lines,said station circuit comprising a coin control mechanism, a switchhookand a dial, mean includin a line finder, a coin control trunk circuit, aselector and a connector all cooperatively responsive to said mechanism,said switchhook and said dial for establishing connections between saidone line and a called subscribers line, a sleeve connec tion in saidline finder, a sleeve circuit in said coin trunk having a first and asecond parallel f branch, a first and a second relay control for saidbranches respectively in said trunk, said first relay responsive to theseizure of said trunk to connect direct ground to said first branch,

the second of said branches extending through continuity contacts ofsaid second relay and through a sleeve connection of said selector todirect ground in said connector responsive to the seizure of a directground connected through a make contact of said continuity contacts tosaid second branch, responsive to the depositing of a coin in said coincontrol mechanism, to prevent double connections and loss of revenue,

3. In a telephone system, in combination in a mechanical switching cointrunk circuit, a tip circuit, a ring circuit, a first polar relay inseries in said tip circuit, a second polar relay in series in said ringcircuit, a third tip and ring reversing relay, a fourth bridge relayhaving a first winding connected to ground and a second windingconnected to battery bridged across said tip and ring circuit when saidtrunk is seized, a

sleeve circuit having a first and a second arallel branch, a fifth and asixth relay control for said branches, respectively, said first branchnormally open at a contact of said fifth relay, said second branchnormally extending through break contacts of a make-before-break contactcombination of said sixth relay, connections between said fourth relayand said fifth relay responsive to the seizure of said trunk to connecta first direct ground to said sleeve, through said first branch, saidpolar relays and said reversing relay selectively responsive tosuccessive steps in the progress of a call through said trunk, and asecond direct ground connectable through the make contact of saidcontact combination to said second branch in parallel with said firstground, responsive to the deposit of a coin in coin mechanism connectedto said trunk, to supplement said first ground to prevent doubleconnections.

4. In a telephone system, a coin trunk circuit for a mechanical system,said trunk having a sleeve circuit, said sleeve circuit having a firstand a second parallel branch, said first branch terminating at an opencontact of a first relay in said trunk when said trunk is idle, saidsecond branch extending through contacts of a second relay in said trunkwhen said trunk is idle, first relay controls for said first relay insaid trunk responsive to the seizure of said trunk to directly groundsaid first branch through said first relay, second relay controls forsaid second relay in said trunk responsive to the deposit of a coin in acoin station circuit connected to said trunk to directly ground saidsecond branch at said contacts of said second relay, said controlsresponsive to the disconnection of said coin station circuit from saidtrunk to restore said branches to their condition when idle.

5. In a telephone system, a first coin-controlled subscriber telephonecircuit, a line finder, a coin trunk circuit, a selector circuit, a,connector circuit and a second subscriber telephone circuit, an operabltelephone path extending between said coin-controlled subscriber circuitand said second subscriber circuit through said line finder, said cointrunk, said selector circuit and said connector circuit, said pathestablishable dependent on the deposit of a coin in coin controlmechanism connected to said coin-controlled circuit, a sleeve circuit insaid trunk circuit, said sleeve circuit extending from a sleeve of saidline finder, said sleeve circuit comprising two parallel branches anddevoid of other branches, one of said branches extending to the opencontacts of a first relay and the other of said branches eX- tendingthrough continuity contact of a second relay, first relay controls insaid trunk, responsive to the seizure of said. trunk by said linefinder, to directly ground said first branch through said contacts ofsaid first relay, second relay controls in said trunk responsive to thedeposit of a coin in said mechanism, for operating said second relay soas to terminate said second branch directly in ground at said continuitycontacts of said second relay, to prevent double connections and loss ofrevenue.

References Cited in the file of this patent UNITED STATES PATENTS Number

